This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The Reaction Center (RC) is the integral membrane protein that performs the light induced electron transfer reactions that are the first steps in conversion of sunlight into chemical energy. Electron transfer occurs from the excited state of a bacteriochlorophyll dimer (D) through several intermediate acceptor molecules to two quinone molecules, QA and QB. The RC forms a complex with another pigment-containing integral membrane protein called the light-harvesting complex (LH1) whose primary function is to gather light and transfer this energy to the RC and photo-oxidize D. In addition, a 14-kilodalton water soluble-cytochrome c2 transiently binds to the RC and re-reduces D. Our ongoing work at SSRL will focus on three structure-function aspects of the RC: 1). Determination of the structure of the RC complexed with other proteins: a). RC and light harvesting protein (LH1), and b). Mutants that affect the binding and electron transfer between RC with cyt c2. 2). Determination of the structures of mutants that affect proton and electron transfer events in RC. 3). Determination of the structures of intermediates that are involved in the charge separation.